The present invention relates to a catalyst activation control system and method for a direct fuel injection engine, and more specifically to a system and a method of activating catalyst disposed in an engine exhaust system by increasing exhaust gas temperature even if the air-fuel ratio of mixture is low. Here, in the direct fuel injection engine, fuel is directly injected (without being mixed with air before induced into each cylinder) into each cylinder and then ignited by spark ignition, respectively.
In the engine in which fuel is directly injected into each cylinder and then ignited by spark ignition, stratified combustion method is generally adopted. In this combustion method, the fuel is injected into each cylinder in the later half period of the compression stroke so that mixture of fuel and air can be stratified and further only a relatively rich mixture in the vicinity of an ignition plug can be ignited, with the result that the engine can be driven in a very lean air-fuel ratio to realize a low fuel consumption.
In the stratification combustion method, however, the mixture formation is severely affected by the fuel atomizing characteristics such as atomization rate and atomization angle of the fuel injector. In more details, FIG. 1 shows the combustion process in the single fuel injection. As shown in FIG. 1, when the atomized fuel is injected at a wide angle, since the outermost gas is excessively lean, even when spark ignited, the outermost fuel cannot be ignited perfectly, so that there inevitably exists a non-combustion region. As a result, the concentration of hydrocarbon (HC) increases in the exhaust gas. Further, FIG. 2 shows the relationship between the heat generation pattern and NO.sub.x generation rate, in the single fuel injection. As shown in FIG. 2, when the amount of the fuel injection is excessively large, the heat generation rate tends to increase in the first half of the combustion, with the result that the gas ignited in the first half is compressed in the second half and thereby nitrogen oxide NO.sub.x tends to be generated in the exhaust gas. This is because the initial combustion becomes active.
Further, in the above-mentioned stratified combustion method of the mixture, the exhaust gas temperature is largely reduced, as compared with the ordinary uniform combustion method. This is because the thermal efficiency is high and thereby the thermal loss is low and in addition the quantity of air heated per unit fuel is large in the stratified combustion. Consequently, the exhaust gas temperature becomes lower than a lower limit of the catalyst activation temperature (which is usually determined on the basis of the exhaust gas temperature of the conventional uniform combustion engine), in particular in a low-load engine driving range such as idling. As a result, there exists such a possibility that the performance of exhaust gas purification deteriorates.
To cope with this problem (i.e., to increase the exhaust gas temperature), Japanese Patent Application laid-open (Kokai) No. 4-183922(1992) discloses such a catalyst activation method that fuel is injected again in the expansion or exhaust stroke of the engine (i.e., the twice fuel injections), in addition to the ordinary single fuel injection, in order to raise the exhaust gas temperature by re-igniting the secondly injected fuel, for activation of the catalyst.
In the above-mentioned prior art method, however, since the fuel is injected and ignited twice for each engine cycle, there exist problems in that the ignition energy consumption not only increases, but also that the possibility of misfire is high in the second ignition.
The reason is as follows:
Since the second fuel is re-injected in the expansion or exhaust stroke and then re-ignited in the exhaust stroke after the primary combustion by the first fuel injection and ignition has been completed, there inevitably exists a time interval between the first primary fuel combustion and the second subsidiary fuel combustion.
Accordingly, it is difficult to form an ignitable mixture in the vicinity of the ignition plug due to a drop in the exhaust gas temperature. That is, it is difficult to well control the second fuel combustion.
In the above-mentioned case, if the second injection fuel is not ignited (the abnormal combustion) the durability of catalyst not only deteriorates markedly, but also the exhaust emission performance is degraded markedly. This is because fuel itself is emitted as it is.
In addition, since the exhaust gas temperature cannot be increased sufficiently high by only the twice fuel injections when the engine is started at a low temperature, it is impossible to activate the catalyst at an early stage. On the other hand, it is not advantageous to always inject fuel twice, from the standpoint of fuel consumption, when the target exhaust gas temperature is determined relatively low according to the engine operating conditions.